1. Go to "Email" -> "Mailing Lists", then "edit" your existing mailing list or create a new one.
2. Click on Membership Management in the top links.
3. Click on Mass Subscription.
4. Choose subscribe for the "Subscribe these users or invite" option.
5. Choose Yes for the "Send welcome messages to new subscribees?" option. Make sure you have your welcome message ready.
6. Choose No for the "Send notifications of new subscriptions to the list owner?" option.
7. Click on Browse to choose the .txt file to upload your members to the new list. The txt file will be a list of your subscribers/members and it has to contain one email address per line.
8. Add your welcome message to the last text field. Make sure you include a couple of blank lines at the end of it.